1. Field of the Invention
This invention relates to regulation of cell growth, and more particularly to regulation of cancer cell growth. In particular, MUC1 peptides derived from a particular region of the MUC1 cytoplasmic domain have been shown to inhibit MUC1 oligomerization and nuclear translocation, causing inhibition and even death of MUC1-expressing tumor cells.
2. Related Art
Mucins are extensively O-glycosylated proteins that are predominantly expressed by epithelial cells. The secreted and membrane-bound mucins form a physical barrier that protects the apical borders of epithelial cells from damage induced by toxins, microorganisms and other forms of stress that occur at the interface with the external environment. The transmembrane mucin 1 (MUC1) can also signal to the interior of the cell through its cytoplasmic domain. MUC1 has no sequence similarity with other membrane-bound mucins, except for the presence of a sea urchin sperm protein-enterokinase-agrin (SEA) domain (Duraisamy et al., 2006). In that regard, MUC1 is translated as a single polypeptide and then undergoes autocleavage at the SEA domain (Macao, 2006).
The MUC1 N-terminal subunit (MUC1-N) contains variable numbers of tandem repeats with a high proportion of serines and threonines that are modified by O-glycosylation (Siddiqui, 1988). MUC1-N extends beyond the glycocalyx of the cell and is tethered to the cell surface through noncovalent binding to the transmembrane MUC1 C-terminal subunit (MUC1-C) (Merlo, 1989). MUC1-C consists of a 58-amino acid extracellular domain, a 28-amino acid transmembrane domain and a 72-amino acid cytoplasmic domain that interacts with diverse signaling molecules (Kufe, 2008). Shedding of MUC1-N into the protective physical barrier leaves MUC1-C at the cell surface as a putative receptor to transduce intracellular signals that confer growth and survival (Ramasamy et al., 2007; Ahmad et al., 2007).
Available evidence indicates that human carcinomas have exploited MUC1 function in promoting tumorigenicity. In this context, with transformation and loss of polarity, MUC1 is expressed at high levels on the entire cell surface in carcinomas of the breast and other epithelia (Kufe, 1984). Other work has shown that overexpression of MUC1 confers anchorage-independent growth and tumorigenicity (Li et al., 2003a; Raina et al., 2004; Ren et al., 2004; Wei et al., 2005), at least in part through stabilization of β-catenin (Huang et al., 2005). Moreover, consistent with a survival function for normal epithelial cells, overexpression of MUC1 confers resistance of carcinoma cells to stress-induced apoptosis (Ren et al., 2004; Yin and Kufe, 2003; Yin et al., 2004; Yin et al., 2007).
Loss of restriction to the apical membrane allows for the formation of complexes with the epidermal growth factor receptor (EGFR) and coactivation of EGFR-mediated signaling (Li et al., 2001; Ramasamy et al., 2007). Overexpression of MUC1 by carcinoma cells is also associated with accumulation of the MUC1-C in the cytosol and targeting of this subunit to the nucleus (Li et al., 2003b; Li et al., 2003c) and mitochondria (Ren et al., 2004; Ren et al., 2006). Importantly, oligomerization of MUC1-C is necessary for its nuclear targeting and interaction with diverse effectors (Leng et al., 2007). For example, the MUC1-C cytoplasmic domain (MUC1-CD) functions as a substrate for c-Src (Li et al., 2001), c-Abl (Raina et al., 2006), protein kinase Cδ (Ren et al., 2002) and glycogen synthase kinase 3β (Li et al., 1998) and interacts directly with the Wnt pathway effector, β-catenin (Yamamoto et al., 1997; Huang et al., 2005), and the p53 tumor suppressor (Wei et al., 2005). Thus, while oligomerization appears to be important, there has been no direct evidence that interference with MUC1 oligomer formation would have any beneficial effects in tumor cells, much less how this might be accomplished.